Localized power point optimizer for solar cell installations

ABSTRACT

In one embodiment, a solar cell installation includes several groups of solar cells. Each group of solar cells has a local power point optimizer configured to control power generation of the group. The local power point optimizer may be configured to determine an optimum operating condition for a corresponding group of solar cells. The local power point optimizer may adjust the operating condition of the group to the optimum operating condition by modulating a transistor, such as by pulse width modulation, to electrically connect and disconnect the group from the installation. The local power point optimizer may be used in conjunction with a global maximum power point tracking module.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to solar cells, and moreparticularly but not exclusively to solar cell installations.

2. Description of the Background Art

Solar cells are well known devices for converting solar radiation toelectrical energy. They may be fabricated on a semiconductor wafer usingsemiconductor processing technology. Generally speaking, a solar cellmay be fabricated by forming P-type and N-type diffusion regions in asilicon substrate. Solar radiation impinging on the solar cell createselectrons and holes that migrate to the diffusion regions, therebycreating voltage differentials between the diffusion regions. In aback-junction solar cell, both the diffusion regions and the metal gridscoupled to them are on the back side of the solar cell. The metal gridsallow an external electrical circuit to be coupled to and be powered bythe solar cell.

A solar cell installation, such as that shown in FIG. 1, typicallyincludes a plurality of solar cells that are strung together to generatepower delivered to a load, such as an inverter. In FIG. 1, theinstallation includes a solar cell module 100 that comprises a pluralityof solar cell strings 104, each with a set of solar cells 102 and adiode D1. The installation may include more than one solar cell module.A maximum power point tracking and regulator module 103 optimizes thepower delivered to the load on a global basis for the entireinstallation regardless of the number of solar cell modules employed.

Each set of solar cells 102 may comprise a plurality of solar cellsarranged in serial fashion. Each diode D1 allows for removal of a string104 from the installation in the event the string 104 is faulty or dragsdown the overall power generation capability of the installation. Duringnormal operation, a substring 104 generates power such that it has thepolarity shown in FIG. 1. In that case, the diode D1 is reversed biasand has no electrical influence on the installation. In the event of afailure, a string 104 changes polarity such that its corresponding diodeD1 is forward biased, thereby shunting off that string 104 from theinstallation.

Although the installation of FIG. 1 and similar installations providemuch needed renewable energy source, techniques for improving theirpower generation capability would make them more economically viable forwidespread use.

SUMMARY

In one embodiment, a solar cell installation includes several groups ofsolar cells. Each group of solar cells has a local power point optimizerconfigured to control power generation of the group. The local powerpoint optimizer may be configured to determine an optimum operatingcondition for a corresponding group of solar cells. The local powerpoint optimizer may adjust the operating condition of the group to theoptimum operating condition by modulating a transistor, such as by pulsewidth modulation, to electrically connect and disconnect the group fromthe installation. The local power point optimizer may be used inconjunction with a global maximum power point tracking module.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a conventional solar cell installation.

FIG. 2 shows a simulated response of a single string of solar cellsoperating at various illumination levels.

FIG. 3 shows a simulated response of three separate strings connected inseries, with one string having various illumination levels.

FIG. 4 schematically shows a solar cell installation in accordance withan embodiment of the present invention.

FIGS. 5-8 schematically show a group of serially connected solar cellsbeing used in conjunction with a local power point optimizer inaccordance with embodiments of the present invention.

FIGS. 9 and 10 show simulated responses illustrating predictedperformance of embodiments of the present invention.

The use of the same reference label in different drawings indicates thesame or like components.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus, components, and methods, to provide a thoroughunderstanding of embodiments of the invention. Persons of ordinary skillin the art will recognize, however, that the invention can be practicedwithout one or more of the specific details. In other instances,well-known details are not shown or described to avoid obscuring aspectsof the invention.

FIG. 2 shows a simulated response of a single string of twenty foursolar cells operating at various illumination levels. In FIG. 2, thevertical and horizontal axes represent the power and voltage outputs ofthe string, respectively. Plot 201 shows the response of the string at 1sun of insolation. Plot 202 is for 0.8 sun of insolation and plot 203 isfor 0.6 sun of insolation. The less than 1 sun of insolation in plots202 and 203 may be attributable to dirt, clouds, shadows, etc. thatblock the sun from fully illuminating all of the solar cells in thestring.

An installation, which in this case comprises a single string, istypically biased to generate a particular output voltage and current(i.e., power). The biasing point is also referred to herein as“operating condition.” It can be seen from FIG. 2 that there is aparticular bias point, referred to as “maximum power point,” where theinstallation generates maximum output power. The power generated by theinstallation diminishes beyond the maximum power point.

FIG. 3 shows a simulated response of three separate strings connected inseries, with each string having twenty four solar cells. In FIG. 3, thehorizontal axis represents output voltage. The left vertical axisrepresents output current and goes with plots 302-304, while the rightvertical axis represent output power and goes with plots 322-324. Inplots 302 and 322, all three strings have the same 100% insolation. Inplots 303 and 323, two strings have 100% insolation and one string has80% insolation. In plots 304 and 324, two strings have 100% insolationand one string has 60% insolation.

Comparing FIGS. 2 and 3, note that a decrease in solar radiation on asingle string on a three-string installation (FIG. 3) impacts theoverall output power significantly more than in a single stringinstallation (FIG. 2). In the case of plots 303 and 323 where one stringis illuminated with 80% sunlight (i.e., 0.8 sun) and the other twostrings in full sunlight (i.e., 1 sun), the installation could provide195 Watts but was providing only 179 Watts. This constitutes a 16 Wattloss in performance. The performance difference becomes more pronouncedwith more severe illumination reduction.

The present invention addresses these problems associated with partialillumination and solar cell performance variations in general byproviding localized power point optimization. A local power pointoptimizer may be configured to manage power generation in each group ofsolar cells in an installation with several groups of solar cells suchthat the entire installation delivers optimum amount of power to theload. Embodiments of the present invention are now described beginningwith FIG. 4. It is to be noted that while the following embodimentsdescribe buck conversion, other approaches, such as boost conversion,linear (continuous) shunting techniques, and AC power conversion, mayalso be used without detracting from the merits of the presentinvention.

FIG. 4 schematically shows a solar cell installation in accordance withan embodiment of the present invention. In the example of FIG. 4, theinstallation includes one or more solar cell modules (also referred toas a “panel”) 401. Each solar cell module 401 may include a plurality ofsolar cells 413, each of which may comprise a back-junction solar cell.The solar cells 413 may be connected in series to form a string. In theexample of FIG. 4, a string 404 includes a group of serially connectedsolar cells 413 and a local power point optimizer 402.

A local power point optimizer 402 may comprise an electrical circuitconfigured to optimize the power contribution of a group of solar cellsto a solar cell installation with several groups of solar cells. A localpower point optimizer 402 may be configured to load a string of solarcells 413 at the point of maximum power delivery (i.e., maximum powerpoint). Nodes 421-426 are shown in FIG. 4 for reference as to how alocal power point optimizer 402 may be included in an installation.

The installation of FIG. 4 also includes a maximum power point tracking(MPPT) and regulator module 414. The module 414 may comprise acommercially-available MPPT module for providing a global maximum powerpoint tracking for the entire installation. The module 414 may beconnected to an inverter of a residential home, commercial structure, orindustrial facility, for example. As will be more apparent below,localized power point optimization in conjunction with global maximumpower point tracking advantageously improves the performance of a solarcell installation.

FIG. 5 schematically shows a group of serially connected solar cells 413being used in conjunction with a local power point optimizer 402A inaccordance with an embodiment of the present invention. Local powerpoint optimizer 402A is a particular embodiment of the local power pointoptimizer 402 shown in FIG. 4. The local power point optimizer 402A maybe serially connected to the installation of FIG. 4 by way of nodes 421and 422, for example.

In one embodiment, the local power point optimizer 402A comprises anelectrical circuit configured to control the amount of power contributedby the serially connected solar cells 413 to the overall installation ofsolar cells. In the example of FIG. 5, the local power point optimizer402A comprises a power point controller 501, a P-channel metal oxidesemiconductor field effect transistor (PFET) Q1, an output stagecomprising an inductor L1 and capacitor C2, a power reserve capacitorC1, a diode D2, and a sensing circuit 503. The sensing circuit 503 maycomprise a resistor, comparator, or other circuit arrangement orcomponent configured to sense the current and voltage output of thestring 404.

The power point controller 501 may comprise electrical circuitconfigured to monitor the voltage and current output of the string 404by way of the sensing circuit 503, determine the maximum power point ofthe string, and control the switching of the transistor Q1 to adjust thebias point of the string to the determined maximum power point. Thepower point controller 501 may employ conventional maximum power pointtracking algorithm, such as the Hill-climbing or RCC algorithm, totrack, and thereby determine, the maximum power point of the string.

In the example of FIG. 5, the power point controller 501 loads the solarcells 413 to adjust the bias point of the string to the maximum powerpoint. In one embodiment, the power point controller 501 does so byadjusting the duty cycle of a pulse width modulated signal used tocontrol the switching of the PFET Q1. Turning ON the PFET Q1electrically connects the solar cells 413 to the rest of the solar cellinstallation. Conversely, turning OFF the PFET Q1 electricallydisconnects the solar cells 413 from the rest of the solar cellinstallation.

The power point controller 501 may be implemented using amicrocontroller to perform maximum power point tracking and a buckregulator to control the loading of the string to adjust its power pointto the maximum power point, for example. The power point controller 501may switch ON the PFET Q1 in situations where the solar cells 413 cangenerate voltage and/or current at the maximum power point, and switchOFF the PFET Q1 in situations where the solar cells 413 cannot generatevoltage and/or current at the maximum power point. The reserve capacitorC1 stores charge from the solar cells 413 when the PFET Q1 is OFF anddischarges when the PFET Q1 is ON. This allows the solar cells 413together with charge stored in the reserve capacitor C1 to delivermaximum power in situations where the solar cells 413 by themselvescould not. The reserve capacitor C1 thus advantageously increases thetime when the solar cells 413 can be connected and thus contributespower to the installation.

As a particular example, assuming the load (e.g., inverter) requests 5amps but the string can only provide a maximum of 4 amps due to a cloudcover or other reasons, the power point controller 501 will adjust theswitching of the PFET Q1 such that approximately 1 amp of current passesthrough the diode D2, bypassing the string. The net impact of this isthat the string would still be providing the installation 4 amps, itsmaximum power point at that time, instead of being in a low-powergenerate state at higher current.

The diode D2 provides a current path through the string when the PFETtransistor is switched OFF. The diode D2 also serves as a safety measurefor shunting the solar cells 413 OFF the installation when the solarcells 413 reverse polarity, such as during very low power generation orfailure. The diode D2 is reversed biased, and therefore does not affectthe operation of the string during normal operation, in that case.

The inductor L1 and capacitor C2 form an output stage for filtering theoutput of the string so that it provides a steady, rather than rapidlychanging, voltage. It is to be noted that the inductor L1 and capacitorC2 are not needed in every string. To save cost and for improvedefficiency, the inductor L1 and capacitor C2 may be limited to some, butnot all, of the strings 404 of the installation of FIG. 4. Depending onthe application, the inductor L1 and capacitor C2 may also be omittedentirely from the installation.

Suitable values for a 5 Amp circuit are about 1 milli Farad forcapacitors C1 and C2 and 10 micro Henry for inductor L1. The capacitorsC1 and C2 preferably have a high ripple current capability (e.g., >5 A)and low ESR. The inductor L1 preferably has low resistance and able tohandle current greater than 6 Amps. The PFET Q1 preferably has as low ONresistance as possible (e.g., STP80 PF55 transistor from STMicroelectronics; 0.016 Ohms ON resistance). A suitable diode D2includes the 80SQ045 diode from International Rectifier. The switchingperiod of the PFET Q1 may be optimized based on selected components, but10-100 kHz may be suitable depending on the application.

FIG. 6 schematically shows a group of serially connected solar cells 413being used in conjunction with a local power point optimizer 402B inaccordance with another embodiment of the present invention. Local powerpoint optimizer 402B is a particular embodiment of the local power pointoptimizer 402 shown in FIG. 4.

The local power point optimizer 402B is the same as the local powerpoint optimizer 402A of FIG. 5 except for the addition of an N-channelmetal oxide semiconductor field effect transistor (NFET) Q2. The NFET Q2provides a low loss (compared to the diode D2) current path through thestring when the PFET Q1 is OFF. The power point controllers 501 and 502are the same except that the power point controller 502 synchronouslyswitches the transistors Q1 and Q2, i.e., PFET Q1 is ON when NFET Q2 isOFF and vice versa. The diode D2 is an optional safety measure in thelocal power point optimizer 402B. The diode D2 may be omitted in someapplications. The components and operation of the power point optimizers402B and 402A are otherwise the same.

The local power point optimizer 402A has fewer components, and is thuscheaper to manufacture, than the local power point optimizer 402B.However, the local power point optimizer 402B is preferable inapplications requiring higher efficiency. A limitation of both localpower point optimizers 402A and 402B is that a P-channel MOSFET, such asPFET Q1, has a minimum resistance of about 0.02 Ohm when ON. This meansthat about 0.5 Watt is wasted by the inefficiency of the PFET. A highercost but higher performance alternative is to use an NFET as a high sideswitch. However, employing an NFET as a high side switch would require avoltage source that is higher in voltage than the output voltage of somesolar cell strings. This necessitates the use of an additional voltagesource, such as a voltage source 702 shown in FIGS. 7 and 8.

FIG. 7 schematically shows a group of serially connected solar cells 413being used in conjunction with a local power point optimizer 402C inaccordance with another embodiment of the present invention. Local powerpoint optimizer 402C is a particular embodiment of the local power pointoptimizer 402 shown in FIG. 4.

The local power point optimizer 402C is the same as the local powerpoint optimizer 402A of FIG. 5 except for the use of an NFET Q3, ratherthan a PFET Q1. The NFET Q3 is coupled to a voltage source 702, whichallows for enough gate voltage to switch ON the NFET Q3. In oneembodiment, the voltage source 702 provides approximately 5V. Thevoltage source 702 may be other strings in the solar cell installation,a voltage converter circuit (e.g., charge pump), a designated solar cellin a solar cell module, etc. The power point controllers 501 and 701 arethe same except that the power point controller 701 provides additionalvoltage to the NFET Q3 to switch it ON. The components and operation ofthe local power point optimizers 402A and 402C are otherwise the same.

The local power point optimizer 402D shown in FIG. 8 is a synchronousversion of the local power point optimizer 402C. A power pointcontroller 801, which is shown in two parts as controllers 801-1 and801-2, synchronously controls the NFET Q3 and NFET Q2 to adjust the biaspoint of the string as before. A voltage source 702 allows the powerpoint controller 801 to apply enough voltage on the gate of the NFET Q3to turn it ON.

The power point controller 801 operates the same way as the power pointcontroller 501 except that the power point controller 801 synchronouslycontrols an NFET Q3 and an NFET Q2. The NFET Q3 is switched OFF when theNFET Q2 is ON and vice versa. Like in the local power point optimizer402B of FIG. 6, the NFET Q2 provides a low loss current path (comparedto the diode D2) when the NFET Q3 is OFF. The diode D2 is an optionalsafety measure in the local power point optimizer 402D. The diode D2 maybe omitted in some applications. The components and operation of thepower point optimizers 402D and 402A are otherwise the same. Suitablevalues for the local power point optimizer 402D for a 5A circuit areshown in Table 1.

TABLE 1 Loss Component Value information Part # C1 3900 uF, 25 V 16mOhms @ UPW1E392MHH 20 C Nichicon C2 1000 uF, 25 V 32 mOhms @UPW1E102MPD 20 C Nichicon Q3 IRF6678 3 mOhms, International Rectifier Qg= 43nC Q2 FDS6690ACT 12 mOhms, Fairchild Semiconductor Qg = 9nCSynchronous FET L1 ~60–120 uH Micrometals T130-53 or similar

FIG. 9 shows a simulated response of a test solar cell module with 30strings, each string having twenty four solar cells. The vertical axisrepresents the maximum power output of the test module in Watts, whilethe horizontal axis represents the performance degradation of a singlestring in percent. In FIG. 9, plot 903 is the response of the testmodule when using a single global maximum power point tracking for all30 strings, with one of the strings being degraded. Plot 902 is theresponse of the test module when using a single global maximum powerpoint tracking for all 30 strings, with one of the strings beingdegraded, and a local power point optimizer 402B (see FIG. 6) for eachof the strings. Plot 902 assumes 99% efficient local power pointoptimizers 402B. Plot 901 is the same as plot 902 in the ideal case,which is 100% efficient local power point optimizers 402B.

As can be seen in FIG. 9, for a single string degradation in excess ofapproximately 8%, a conventional maximum power point tracking approach(plot 903) does not recover the maximum power available from the testmodule. In fact, when the individual string has a reduction inperformance in excess of approximately 15%, the conventional approachutilizes the overall test module at a bias point where the degradedstring provides zero power. In contrast, local power point optimization(plots 902 and 901) ensures that all strings in the test module alwaysprovide power to the load, independent of how degraded their performancemight be relative to others. As a point of reference, 100% efficientlocal power point optimization (plot 901), the ideal case, ensures thatthe safety diodes (e.g., diodes D2) are never forward biased.

FIG. 10 shows a simulated response of the test solar cell module with 30strings, each string having twenty four solar cells. The vertical axisrepresents the recoverable power of the test module in Watts, while thehorizontal axis represents the performance degradation of two strings inpercent. In FIG. 9, plot 906 is the response of the test module whenusing a single global maximum power point tracking for all 30 strings,with two of the strings being degraded. Plot 905 is the response of thetest module when using a single global maximum power point tracking forall 30 strings, with two of the strings being degraded, and a localpower point optimizer 402B for each of the strings. Plot 905 assumes 99%efficient local power point optimizers 402B. Plot 904 is the same asplot 905 in the ideal case, which is 100% efficient local power pointoptimizers 402B.

As can be seen from FIG. 10, for performance reduction of 20% in two ofthe thirty strings in the test module, 99% efficient local power pointoptimization (plot 905) can extract 90 W (110 W−20 W) more from the testmodule compared to the conventional approach (plot 906).

While specific embodiments of the present invention have been provided,it is to be understood that these embodiments are for illustrationpurposes and not limiting. Many additional embodiments will be apparentto persons of ordinary skill in the art reading this disclosure.

1. A solar cell installation comprising: a plurality of groups of solarcells, the plurality of groups of solar cells being seriallyinterconnected to generate an overall output power to a load; a separatelocal power point optimizer for each group of solar cells in theplurality of groups of solar cells and serially interconnects acorresponding group of solar cells to other groups of solar cells in theplurality of groups of solar cells, the local power point optimizerincluding a power point controller configured to modulate a firsttransistor to electrically connect and disconnect the correspondinggroup of solar cells to the installation to adjust a bias point of thecorresponding group of solar cells to deliver maximum power asdetermined by the local power point optimizer; and a maximum power pointtracking module configured to adjust an operating point of the pluralityof groups of solar cells to maximize the overall output power to theload as determined by the maximum power point tracking module; whereinthe separate local power point optimizer for each group of solar cellsin the plurality of groups of solar cells further comprises an outputstage at the output of the separate local power point optimizer, theoutput stage being configured to filter a voltage output of thecorresponding group of solar cells.
 2. The solar cell installation ofclaim 1 wherein the local power point optimizer synchronously controlsthe first transistor and a second transistor to adjust the bias point ofthe corresponding group of solar cells, the second transistor providinga current path through the corresponding group of solar cells when thefirst transistor is switched OFF.
 3. The solar cell installation ofclaim 2 wherein the first transistor comprises a PFET and the secondtransistor comprises an NFET.
 4. The solar cell installation of claim 1wherein the output stage comprises an inductor having a first end and asecond end and a capacitor having a first end and a second end, thefirst end of the inductor being coupled to the first transistor, thesecond end of the inductor being coupled to a first end of thecapacitor, the second end of the capacitor being coupled to anotherlocal power point optimizer of another group of solar cells in theplurality of solar cells.
 5. The solar cell installation of claim 1wherein the first transistor comprises a PFET.
 6. The solar cellinstallation of claim 1 wherein the first transistor comprises an NFET.7. The solar cell installation of claim 6 further comprising a voltagesource configured to provide additional voltage to a gate of the firsttransistor to allow the first transistor to be switched ON.
 8. The solarcell installation of claim 1 further comprising a diode across thecorresponding group of solar cells, the diode being configured to bereversed bias during normal operation of the corresponding group ofsolar cells.
 9. The solar cell installation of claim 1 furthercomprising a capacitor configured to be charged by the correspondinggroup of solar cells when the first transistor is switched OFF.
 10. Asolar cell installation comprising: a plurality of seriallyinterconnected groups of solar cells; an electrical circuit in eachgroup of solar cells in the plurality of serially interconnected groupsof solar cells, the electrical circuit serially interconnecting acorresponding group of solar cells to other groups of solar cells in theplurality of serially interconnected groups of solar cells, theelectrical circuit being configured to determine an operating conditionof the corresponding group of solar cells where the corresponding groupof solar cells generates a target power output and to adjust thecorresponding group of solar cells to operate at the determinedoperating condition, the electrical circuit in each group of solar cellsin the plurality of serially interconnected groups of solar cellsfurther comprising an output filter stage, the output filter stagecomprising an inductor at the output of the electrical circuit.
 11. Thesolar cell installation of claim 10 wherein the electrical circuitmodulates a first transistor to electrically connect and disconnect thecorresponding group of solar cells from the installation such that thecorresponding group of solar cells generates the target power output.12. The solar cell installation of claim 11 wherein the electricalcircuit synchronously switches the first transistor and a secondtransistor to electrically connect and disconnect the correspondinggroup of solar cells from the installation such that the correspondinggroup of solar cells generates the target power output.
 13. The solarcell installation of claim 12 wherein the first transistor comprises aPFET and the second transistor comprises an NFET.
 14. The solar cellinstallation of claim 11 wherein the first transistor comprises an NFET.